Marathon Monitors Inc.
AACC 2000 Carbon Nov. 1, 1997
58
Integral action and manual reset
In a full three-term controller (that is, a PID controller), the integral term ‘ti’
automatically removes steady state errors from the setpoint. If the controller is set up to
work in two-term mode (that is, PD mode), the integral term will be set to ‘OFF’. Under
these conditions the measured value may not settle precisely at setpoint. When the
integral term is set to ‘OFF’ the parameter manual reset (code ‘rES’) appears in the
‘Pid LiSt’ in ‘FuLL’ level. This parameter represents the value of the power output
that will be delivered when the error is zero. You must set this value manually in order
to remove the steady state error.
Automatic droop compensation (Adc)
The steady state error from the setpoint, which occurs when the integral term is set to
‘OFF’ is sometimes referred to as ‘droop’. ‘Adc’ automatically calculates the manual
reset value in order to remove this droop. To use this facility, you must first allow the
process variable to stabilise. Then, in the autotune parameter list, you must set ‘Adc’ to
‘on’. The controller will then calculate a new value for manual reset, and switch ‘Adc’
to ‘OFF’.
‘Adc’ can be repeated as often as you require, but between each adjustment you must
allow time for the process variable to stabilise.